During the battle with Salabog, it will occasionally summon small groups of Will o' the Wisps to aid it in attacking you. Because of the narrowness of the area during this battle, it is advisable to get rid of them as quickly as possible, so as not to be unnecessarily injured and potentially delayed by them when trying to score a hit on Salabog itself. With their low HP and defense, the Dog can take care of them while the player avoids them, if they have the room to.
Will o' the Wisps offer almost no money or experience. They are not at all useful for levelling up.